Common corridors in residential building

1. I consider them residential as access to the room is their sole purpose.

2. NA

For ASHRAE 90.1-2010 Appendix G:

When modeling corridors, how is G3.1.1 exception a being applied when the common area is larger than 20,000 ft2?

Use additional system type(s) for nonpredominant conditions (i.e., residential/ nonresidential or heating source) if those conditions apply to more than 20,000 ft2 of conditioned floor area.

Also 9. Thermal Blocks -- Multifamily Residential Buildings

Residential spaces shall be modeled using at least one thermal block per dwelling unit, except that those units facing the same orientations may be combined into one thermal block. Corner units and units with roof or floor loads shall only be combined with units shoring these features.

In Table G3.1.1A Baseline HVAC systems types in the note, defines Residential spaces Residential building types include dormitory, hotel, motel, and multifamily. Residential space types include guest rooms, living quarters, private living space, and sleeping quarters. Other building and space types are considered nonresidential.

Where attributes make a building eligible for more than one baseline system type, use the predominant condition to determine the system type for the entire building except as noted in Exception a to Section G3.1.1.

If the common area spaces in the building are more than 20,000 ft2, the corridors should be modeled with a different system and not the residential building type system.

Similar language is in ASHRAE 90.1-2004, 2007, 2013, 2016 and 2019.

It's not - if the only occupancy on those floors is residential then the group of floors is looked at together as a "residential building" without regard to some minority subset of the spaces. 

Where is a building defined by “individual floors”? 

Exceptions in G3.1.1 are for the entire building, not individual floors. This is not in ASHRAE 90.1 or any other code. ASHRAE 90.1 defines both “Residential building types” as well as “Residential spaces”, they also define "Residential". Corridors are not listed in these definitions.

For "multifamily residential buildings", the corridor is considered a commercial space, due to it being a path of egress for not just one dwelling unit, but all the units on the floor.  Within Section 9: Lighting, corridors have a lighting power density and lighting control requirements whereas dwelling units themselves are exempt from a lighting power density within ASHRAE 90.1 per section 9.1.1.

Even the definition within ASHRAE 90.1 defining residential: spaces in buildings used primarily for living and sleeping. Residential spaces include, but are not limited to, dwelling units, hotel/motel guest rooms, dormitories, nursing homes, patient rooms in hospitals, lodging houses, fraternity/ sorority houses, hostels, prisons, and fire stations.

A corridor's primary use is not for living and sleeping.  A corridor is a transitory space used for ingress and egress from primary space types.

GBCI has typically allowed for flexibility in classifying residential corridors as either residential (since the footnote to Table G3.1.1A classifies certain building types such as dormitory, hotel, motel and multifamily as residential) or nonresidential (since the corridors are not in the examples of residential space types in the definitions or the footnote to Table G3.1.1A). 

See the example that is described by PNNL. The corridor is considered as common space for the residential building. The examples are on page 41 (3.8).  https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-2...